Cerebrospinal fluid is formed in the ventricular system irrespective of the intracranial pressure (ICP). The formation rate is constant, with a range of 0.3-0.4 ml/min. (Børgesen and Gjerris 1987). Hydrocephalus, i.e. a pathological increase in the amount of intracranially located cerebrospinal fluid, arise when the outflow of the cerebrospinal fluid is obstructed, leading to an increase in the intracranial pressure and in the amount of intracranially located cerebrospinal fluid. The obstruction may be localized in the aqueduct or the IV ventricle or in the normal resorption sites in villi arachnoidales in connection with the sagittal sinus. Patho-anatomically, hydrocephalus is divided in communicating or non-communicating hydrocephalus dependent whether there is passage between the ventricular system and sinus sagittalis or not. Communicating hydrocephalus, which is generally caused by obstruction located in the villi arachnoidales for example due to fibrosis formed in response to bleeding in the liquor, is the most common form of hydrocephalus.
The treatment of hydrocephalus aims at reducing the intracranial pressure to normal, physiological values and thereby also reducing the amount of cerebrospinal fluid towards normal, physiological values. This is obtained by deducting cerebrospinal fluid (CSF) from the ventricular system to another resorption site, bypassing the pathological obstruction by use of a CSF shunt. The most suitable diversion sites have been found to be the right atrium of the heart and the peritoneal cavity. Valves have been designed to hinder retrograde flow in the drainage system which could occur due to pressure differences between the intracranial cavity and the resorption site, e.g. in connection with increased chest and/or abdominal pressure in connection with e.g. cough or defecation.
Until the last 6 years the CSF shunts have been based on principle of maintaining a constant ICP regardless of the flow-rate of CSF. The CSF shunts have been constructed to off CSF-flow when the differential pressure between the in and the outlet of the CSF shunt was reduced to a predestined level, called the opening pressure of the shunt. This has been necessary in order to maintain a basal ICP due to the use of an unphysiological resorption sites located outside the intracranial cavity. Example of a such ICP shunt is shown in U.S. Pat. No. 4,904,236 which is a fluid flow control device for controlling the flow of fluid from one region of the body be drained to another region.
Clinical experience has proven that this principle of shunting is not an ideal solution. Sudden rises of the ICP, i. e. due to change of position, physical exercise, or pathological pressure waves result in excessive CSF drainage. This so-called hyperdrainage leads to subnormal ICP for shorter or longer periods of time. Several reports in the literature (Aschoff et al., 1995) point at problems due to this hyperdrainage, and especially the pronounced narrowing of the ventricles has been pointed out to be the main factor leading to malfunctioning of the implanted shunting device. The reason is that the ventricular walls may collapse around ventricular CSF shunt device, and particles (cells, debris may intrude into the shunt device.
This has led to the introduction of multiple designs of drains to be used in the ventricular cavity. An effect of these different drain designs on the complication rates of shun has not been proven.
In recent years, CSF shunt devices have been introduced which aim at regulating the flow rate of CSF, see e.g. U.S. Pat. No. 4,781,673 which describes a brain ventricle shunt system with flow rate switching means.
An alternative flow regulating mechanism of the Orbis Sigma shunt results in partial closure of the shunt at increases of the differential pressure above 10 mm Hg, and in reopening of the shunt when the differential pressure exceeds 35 mm Hg. It has been shown that this type of shunt indeed leads to a reduction of the complication rate of the system. Another shunt system, The Pudenz Delta valve, also hinders excessive CSF outflow at higher-pressure levels. U.S. Pat. No. 4,605,395 is an example of a shunt device comprising a non-linear hydraulic filter valve which closes in the event of large changes in flow rate.
Still, the above CSF shunt systems drain the CSF to a resorption site that is far from normal and to a site where the pressure difference over the shunt may differ substantially from the normal, physiological pressure ranges.
Occasional reports in the literature have described the use of ventriculo-superior sagittal shunts for the treatment of hydrocephalus (Hash et al., 1979 and Wen, 1981). In the article by Hash et al. it is concluded that the described technique wherein a low-low or extra-low pressure one way valve is used may be suitable for patients with high pressure hydrocephalus and of particular value in very ill or debilitated patients because of the rapidity with which it can be performed under local analgesia whereas its use in normal low pressure hydrocephalus must still be evaluated. This article is followed by a comment by the editor that there are a multitude of remaining critical questions. One of the problems not addressed in this study is overdrainage due to the fact that the used valve is not flow-restricting.
Wen et al., 1981, reports the treatment of fifty-two children with hydrocephalus with ventriculo-superior sagittal sinus shunts by use of a modified Pudenz tube. In this tube there is provided slits which provide an opening pressure of about 6 mm Hg. No clear conclusion can be drawn from this report except that shunting to the sagittal sinus does not inherit serious complications.
U.S. Pat. No. 5,000,731 describes a drain consisting essentially of a thin film and a ventricular tube having an open end and a closed bottom end for shunting cerebrospinal fluid to the subdural space on the surface of the brain. It is intended that through arachnoid lacerations or openings during the shunting procedure, the CSF in the subdural space will then enter into the subarachnoid space and be further absorbed by the arachnoid villi. Although this device has the benefit of being an intracranially located shunting device, it is draining the cerebrospinal fluid to an unphysiological place as it should be noted that under normal physiological conditions the subdural space is a potential space only which has gained its name due to the pathological occurrences of e.g. subdural haematoma which can occur in connection with lesions of the vascular system. Moreover, this system is only applicable in patients with normal resorption at the sagittal sinus, i.e. non-communicating hydrocephalus.
EP 066 685 describes a drain comprising a bundle of one or more microtubules, each being about 0.44 mm in diameter for controlling hydrocephalus comprising a plurality of pliable microtubular members for conducting cerebrospinal fluid from the cerebral ventricle to selected areas of the human body, e.g. to the subarachnoid space. Essentially, this patent relates to a draining system aiming at avoiding obstruction due to clotting of the draining system and is not flow-regulating.
WO 98/11934 describes a cerebrospinal fluid shunt system which drains surplus CSF to the sagittal sinus by means of a shunt with in-built resistance equal to normal values for CSF outflow-resistance and a unidirectional valve. It has surprisingly been found that this type of shunt drains insufficiently in patients with so-called normal pressure hydrocephalus. While functioning correctly, as measured by testing the inserted shunt, the shunt has failed to relieve the clinical symptoms in some of the shunted patients suffering from normal pressure hydrocephalus.
In normal pressure hydrocephalus a balance between the intracranial pressure and the stress on the ventricular walls has reached an equilibrium. The dilatation of the ventricles is followed by a decrease in the pressure necessary to maintain the dilatation, cf. the law of LaPlace (S.E. Børgesen et al., 1987).
Pressure waves (B-waves) still occur, but the amplitude is low. The resistance to outflow in this condition is still above the normal level of around 10 mm Hg/ml/min. In this condition, a drainage with a ventriculo-sagittal shunt with a resistance of 8-10 mmHg/ml/min will not lead to a decrease in the size of the ventricles. The low pressure necessary to maintain the stress on the ventricular walls means that the differential pressure from the ventricles to the sagittal sinus is very low, resulting in insufficient drainage of the surplus CSF. B-waves, which occur in the condition of normal pressure hydrocephalus will result in short time increases of the intracranial pressure, but a resistance to outflow above 8 mm Hg/ml/min means that only a fraction of the needed CSF drainage takes place. In this condition, shunts with a resistance to outflow in the range of 4-8 mm Hg/ml/min will be needed.
The same will be the case in children with very large ventricles, where the intracranial pressure may be too low to allow for a sufficiently pressure difference to establish sufficient CSF drainage.